Brine Drilling Fluid
Polymers are added to brine to viscosify the water and provide some filtration control. Certain polymers (XC or Duovis) are of particular value since they possess low viscosity at high shear rate, and high viscosity at low shear rates. The effect of this is good flow properties in the drillstring (at high shear rate) combined with good lifting properties in the annulus (low shear rates). About 0.5 lb/bbl of XC polymer should be added. Drilled solids must be controlled by dilution and mechanical devices. Good performance is achieved using desanders and desilters.
An oil-based mud is one in which the base fluid from which the mud is made up is oil. Since the 1930’s it has been recognised that better productivity is achieved from reservoirs when oil based fluids rather than water based fluids are used to drill through the reservoir. This is largely because the oil does not cause the clays in the reservoir to swell or cause changes in wettability of the formations. Crude oil was first used to drill through the pay zone, but it suffered from several disadvantages (low gel strength, limited viscosity, safety hazard due to low flash point). Modern oil-based muds use low-toxicity base oils and a variety of chemical additives to build good mud properties. The use of oil in the drilling fluid does have several disadvantages:
• Higher initial cost
• More stringent pollution controls required
• Reduced effectiveness of some logging tools (resistivity logs)
• D etection of kicks more difficult due to gas solubility in base oil
However for some applications oil-based muds are very cost effective. These include:
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To drill and core pay zones
To drill troublesome formations (e. g. shale, salt)
To add lubricity in directional drilling (preventing stuck pipe) To reduce corrosion
As a completion fluid (during perforating and workovers)
There are three types of oil-based muds in common use:
• Full oil (water content < 5%)
• Invert oil emulsions (water content 5 — 50%)
• Synthetic or Pseudo oil based mud
The first oil base drilling fluid was crude oil, and was used to complete shallow, low pressure zones. Although there is no record of its first usage, it probably occurred soon after the advent of rotary drilling. The first patent application for an oil base drilling fluid was issued in 1923, but this fluid was not a commercial success.
Oil Base Drilling Fluids Company (now Hughes Drilling Fluids) was formed by George Miller to manufacture, market, and service the first commercial oil base drilling fluid, Black Magic. On May 1, 1942, Richfield Oil Company (now ARCO) used Black Magic as a completion fluid. Black Magic at that time was composed of air blown asphalt dispersed in a diesel oil which contained naturally occurring naphthenic acid, quick lime, and 5% by volume water. The uses of Black Magic in these early years were as completion fluids for low pressure and/or low permeability sands, coring fluids, and to free stuck pipe.
This original system performed well when applied properly. However, it had some obvious drawbacks. Asphalt was the primary viscosifier and fluid loss control additive. It did a good job of both but contributed to very high apparent and plastic viscosities and consequently was detrimental to drilling rates when compared to a water mud of the same density. It was also much more expensive per unit volume than water mud.
Because it did perform many functions well, the industry then set about to improve on it. From this work came the development of what are called the Inverts or Invert Emulsion Muds. Invert emulsion means that water is emulsified in oil (water-in-oil emulsion). In the earlier years (1940’s), one of the most popular water muds run was oil-in-water. These muds were called oil emulsion systems. Therefore, during the development of invert emulsion systems, the term "inverts” or invert emulsion was used to differentiate the oil system containing some oil.
The control of the water base muds is made possible because of the wide variety of additives available for performing specific functions. At this time in history, development of oil mud additives and the technology of oil muds were pointed in the same direction. The first step dealt with the amount of water emulsified. Inverts were developed to contain and tolerate a much greater water volume than true oil muds. Rheology could then be controlled by altering oil/water ratios. This allowed the system to have adequate weight material suspension and filtration control with lower viscosity and gels. Water contamination became a less acute problem with inverts. Oil/water ratios ranged from 55/45 to 70/30.
The initial preparation of many oil muds tended to be time consuming and expensive because additives such as asphalt did not blend readily in crude or diesel oils but required heat for adequate dispersion. Muds containing these additives had to be prepared at a mixing plant and hauled to the rig site. Make up costs were also high with true oil mud due to higher volume percentage of oil plus the large additions of asphalt.
Water contamination was an acute problem causing excessive viscosity and water — wetting of solids, necessitating replacement of the system or at least dilution with new mud. Water contamination of invert emulsions required adjustment of mud properties by the addition of oil and emulsifiers. The principal components in the oil muds could not be added to adjust a single property without affecting most of the other mud properties. Single additives to adjust or control specific mud properties were not available at the time to provide the flexibility and versatility needed for lower cost.
The original inverts were composed of the same basic ingredients as the true oil muds. The concentrations of materials differed however. Calcium and magnesium soaps were used along with asphalt in small concentrations. Sodium chloride brine was used as the internal phase. The earliest of these systems, No-Blok (Magcobar) and Kenex (Ken Corp., later IMC) did not have any other additives. Although they were more flexible (rheologically) than the true oil mud, they were not as stable.
In recent years the base oil in OBMs has been replaced by synthetic fluids such as esters and ethers. These fluids are generally called synthetic or psuedo oil based muds.